Four people including a police officer were killed and 40 injured when a knife-wielding terrorist brought horror to the gates of Westminster.

The attacker mowed victims down on Westminster Bridge before crashing his car into railings beneath Big Ben and rushing at police guarding parliament through an open gate. He stabbed and killed an officer, named as Keith Palmer, 48, inside New Palace Yard before being shot dead.

Prime Minister Theresa May was ushered away from parliament by her security detail after the attack.

Speaking outside 10 Downing Street after chairing a meeting of the government’s Cobra emergencies committee, Mrs May paid tribute to the “exceptional men and women” of the police force who responded to the attack.

She vowed that “the forces of evil” would never be allowed “to drive us apart”.

Theresa May has branded the attack 'sick and depraved'.

Police said they were treating the incident, which began at about 2.40pm, as a terrorist attack.

Metropolitan Police Acting Deputy Commissioner and Head of Counter Terrorism Mark Rowley said: “This was a day we’ve planned for but hoped would never happen. Sadly it is now a reality.”

At Holyrood, where MSPs were due to vote on a call for a second Scottish independence referendum, parliament was suspended following the attack. First Minister Nicola Sturgeon said her thoughts were “with everyone caught up in the dreadful incident at Westminster”.

This was a day we’ve planned for but hoped would never happen. Sadly it is now a reality.

Mark Rowley

The attacker left a trail of destruction for hundreds of metres from Westminster Bridge to parliament. Victims hit by the Hyundai 4x4 were described as having “catastrophic injuries” by staff at St Thomas’ Hospital across the Thames, who rushed to aid those hurt.

One woman hit by the attacker’s car was confirmed dead. Another woman who fell into the Thames was rescued and given urgent medical treatment on a pier.

A party of French schoolchildren were among those targeted by the car, with three injured. London Ambulance Service said paramedics treated at least ten patients on Westminster Bridge.

On the floor of the cobbled courtyard in front of Parliament, paramedics fought to save the life of the police officer and his attacker, with Foreign Office minister Tobias Ellwood among those who rushed to help.

Kirsten Hurrell, who runs a newspaper kiosk in Parliament Square, told journalists that the vehicle was shot at before it careered into the fence outside parliament.

She said: “There were shots to stop the car, the car turned, went completely over the cycle lane and rammed nose first into the parliamentary fence. There was someone on the ground on the right of the car.”

Another eyewitness, Jayne Wilkinson said: “We were taking photos of Big Ben and we saw all the people running towards us, and then there was an Asian guy in about his 40s carrying a knife about seven or eight inches long.

“He was running through those gates, towards Parliament, and the police were chasing him.”

A policeman points a gun at a man on the floor at the top of the frame as emergency services attend the scene outside the Palace of Westminster. Picture: PA

Frazer Clarke, 25, from Burton-on-Trent, one of a group of young boxers who saw the attack, said: “We heard a loud bang and screaming and then I noticed some smoke. I thought it was a car crash.

“I looked towards the front gate and people were running, a police officer and a fellow coming to the gate with two knives.

“He was stabbing the police officer with the knives.

“The police officer was stumbling and fell on the floor.

“Another police officer walked toward the guy with the knives and shot him twice.”

Edinburgh South Labour MP Ian Murray said he had been heading for a vote at the time of the incident. He added: “Anti-terror armed police were everywhere. Security and emergency services have been unbelievable.”

He said he had just been attending a meeting of an all-party parliamentary group.

“I went back up to them to let them know what was going on and make sure they didn’t leave the room we were in.

“Since, we have been moved to several places and locked down - currently in Westminster Abbey.”

MPs spent hours in lockdown in the House of Commons chamber as police initially held thousands of people in place before evacuating them from the parliamentary estate.

From inside the chamber, SNP MP Ian Blackford told The Scotsman: “We at least have benches to sit on - but our thoughts are with those who give us security and who put themselves in harm’s way just by doing their jobs.”

Police Federation chairman Ken Marsh expressed “deepest sympathy” to the family of the officer who died following the events of Wednesdayyesterday afternoon.

He added: “This incident sadly shows the dangers our colleagues face on a daily basis. They show incredible bravery protecting the public.”

Labour leader Jeremy Corbyn MP, Leader of the Labour Party, said: “Today’s horrific attack in Westminster has shocked the whole country. All our thoughts are with the victims of this outrage, their loved ones, families and friends.

“This was not only an attack on innocent people.

“It was also an attack on our democracy. But I know that Londoners and people across the country will stand together in defence of our values and diversity.”

Liberal Democrat leader Tim Farron said: “When the attacks happened, many people, both police and civilians, ran towards the danger in order to help. Their bravery was awe-inspiring.

“Our response is to this must be more democracy, more openness, and more humanity. We will answer hatred with love.”

The Scotsman’s office at Westminster overlooks New Palace Yard, where the incident unfolded.

Correspondent Paris Gourtsoyannis said: “There was a loud bang, like a car accident, from the direction of Whitehall and Westminster Bridge, to the east of parliament.

“Passers-by and police officers manning the gates were beginning to move towards the sound, when there were shouts, and what looked like someone rushing into New Palace Yard.

“There was a melee of police officers inside the gates, and then the sound of at least two gunshots.

“Two people were left lying on the pavement, which loops around to the underground parking lots.

“It wasn’t clear what the assailant was armed with. Officers immediately began giving medical attention to both of the injured, with paramedics giving CPR.”

Mayor of London Sadiq Khan said: “There has been a serious incident near to Parliament Square this afternoon which is being treated as a terrorist attack until the police know otherwise.

“I have spoken to the Acting Commissioner. The Metropolitan Police Service is dealing with the incident and an urgent investigation is under way. My thoughts are with those affected and their families.

“I would like to express my thanks to the police and emergency services who work so hard to keep us safe and show tremendous bravery in exceptionally difficult circumstances.”